The subject invention relates to an apparatus and method for printing images. More particularly, it relates to an offset printing apparatus incorporating an ink jet for producing images on a transfer roller for printing variable images such as postal indicia printed by postage meters to evidence that appropriate postage has been paid on a mail piece.
As evidence that postage has been paid, i.e. that the prepaid amount stored in the meter has been properly decremented, a postage meter will print a postal indicia on a mail piece. Typically, indicia have been printed by complex mechanical rotary or flat bed printing elements which include a fixed printing element for printing fixed information and adjustable elements for printing variable information.
Such meters have been highly successful and are presently used to account for the expenditure of billions of dollars of postage in the United States alone. However, in an effort to increase the security of these funds above the present high level, the U.S. postal service has proposed a standard which would require that the indicia include encrypted information which would vary from indicia to indicia so that counterfeit indicia could not be produced without knowledge of the encryption method and key used.
Additionally it is very advantageous to provide users of postage meters with the ability to print ads or slogans along with the postal indicia, and it would be highly desirable to enable these users to easily vary such ads or slogans.
In response to these needs the applicant has proposed an offset printing apparatus which incorporates an ink jet for producing images on a transfer roller. This printing apparatus is more fully described in commonly assigned co-pending U.S. patent applications Ser. Nos. 08/751,290 and 08/751,663, filed on even date herewith. (E-519 and E-557)
While it is expected that these apparatus will provide very substantial advantages in the printing of variable images such as postal indicia, some problems are anticipated. As is common with all ink jets conventional maintenance operations to prevent clogging of the ink jet nozzles are necessary. Further, as the number of print cycles increases it is expected that areas of dried and partially dried ink may form on the transfer roller; causing smearing on the envelope or other substrate.
Thus, it is an object of the subject invention to provide an improved method for maintenance of a printing apparatus which incorporates an ink jet for producing images on a transfer roller.